Tuning selectivity control circuit



April 13, 1937.

P. o. EARNHAM TUNING SELECTIVITY CONTROL CIRCUIT Filed May 25, 1955 RN'FTAM INVENTQR ATTORNEY 0%. E325 m L .w N #33 m AU") [I] v QEENGR "H m P m K323 EEQ Qu E n 11111 m N \vm m 1 5 v m llll & Etkuwm r n l b T I- 11:. |..J &. H ank m N 4. WWW. l a ll H 0 2W 5W W m w r H 0 Lisa w i m REDS Q QEEEQ Q Patented Apr. 13, 1937 TUNING SELECTIVITY CONTROL CIRCUIT Paul 0. Farnham, Boonton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 25, 1935, Serial No. 23,445

2' Claims.

I My present invention relates to tuningcircuits for radio receivers, and more particularly to novel circuits for controlling the tuning of superheterodynereceiving systems so as to increase the apr 1.; 5 parent sharpness of tuning.

In radio receivingsystems of the superhetero- -dyne type, the latter being substantially universally employed for radio reception at the pres- ,ent time, it is desirable to have the signal se- 11 ,lector network of the system increase in apparent selectivityas the system is tuned to resonance with a given broadcasting station. This is especially true of high fidelity receivers, since in the latter the listener finds it particularly ane3 noying to listento. distortion fringes ofthe accepted band of frequencies. The ideal selectivity characteristic of a receiver is one wherein the response of the system is. substantially zeroat the limits of the desired band, and the sides of the resonance curve are steep and substantially parallel; the band being sufficiently wide, of course, to substantially uniformly transmit tothe demodulator all the reproducible audio frequencies.

It may, therefore, be stated that-it is an important object of this invention to provide a selectivity control network for a radio receiver of the superheterodyne type wherein the sharpness oftuning of the receiver is increasedby phase opposition of intermediate and signal carrier frequency effects through a double function stage. ,Another important object of this invention is ,to provide in a superheterodyne type of receiver a frequency changer network disposed between ;.the signal input element and the intermediate frequency amplifier, the frequency changer network comprising a pair of tubes arranged in cascade and each ofthem being coupled to a local oscillator so that each tube acts as a first detector; signal and intermediate frequency circuits being arranged between the cascaded tubes to increase the apparent sharpness of tuning of the receiver as it is tuned through the tuning range. 9

Still other objects of the invention are to improve generally the efliciency and tuning of superheterodyne receivers, and more especially to provide improved tuners in an economical and simple manner in such receivers.

The novel features which I believe to be. characteristic of my invention are set forth in particularity'in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

In the drawing:

Fig. 1 is a circuit diagram of a superhetero- 'dyne receiver-embodying the invention,

Fig. 2 graphically shows the effect of the invention.

Referring now to the circuit diagram in Fig. 1, it is to be noted that the receiving system shown includes conventional networks with the exception of that portion of the system which embodies the invention. The receiver is a superheterodyne of the multi-range type, and includes a signal collector A which may be the usual grounded 15 antenna, a loop, or any other known type of collector. While the collector may be followed by one, or more, stages of tunable radio frequency amplification prior to the first detector, the signal amplifiers have been omitted to preserve simplicity of disclosure. The first detector network comprises a pair of cascaded tubes I, 2; each of these tubes being of the pentode type and being fed with local oscillations from a local oscillator 3. The intermediate frequency energy produced 25 in the output circuit 4 of tube 2 is amplified in the I. F. amplifier 5. The output of the latter is demodulated in a second detector 6; the demodulated signal energy is then amplified in one, or

more, audio amplifiers, and finally reproduced in any desired type of reproduoer.

Th usual AVC network is employed to overcome fading effects; the volume control arrangement is schematically represented since those skilled in the art are fully aware of the manner of constructing such an arrangement. For example, the AVC arrangement shown by Stuart Ballantine in application Serial No. 376,163, filed July 5, 1929, Patent No. 2,046,237, June 30, 1936, may be used. The control bias produced by the AVG rectifiermay'be used to vary the gain of the I. F. amplifier 5, and also the gain of each of the first detector tubes l and 2. The lead 1,

designated AVG, is connected to the signal grids of tubes I, 2 and the grid circuits of the tubes of network 5. v V

Considering, now, the networks including the present invention, the tube I functions as a first detector to produce 1. F. energy and as an amplifier of signal energy. The signal input circuit comprises a coil 8 and a variable tuning condenser 9 connected in parallel. Thecoil 8 is tapped to provide several tuning ranges, and the tap H1 is adjusted to change the operating tuning range of the set. This is a well known and conventional device, and need not be described any further. The collector A and the signal grid of tube l are connected to the high alternating potential side of the signal input circuit. The low alternating potential side of coil 8 is grounded through a condenser The AVC lead I is connected to the the junction of coil 8 and condenser through a filter resistor l2.

The cathode of the tube I has the locally produced oscillations of the local oscillator 3 impressed upon it. The oscillator is of a conventional type, and has its anode regeneratively coupled to the low alternating potential side of coil l3 through condenser M. The coil I3 is provided with the range switching device described above. The oscillator tuning condenser l5 has its rotor mechanically coupled with the rotor of the condenser 9. The cathode circuit of oscillator tube 3 includes the grounded bias resistor Hi; the cathode side. of resistor I6 is connected by lead II to the cathode of tube Thus, in the absence of AVG action, normal signal grid bias for tubes and 2 is derived from the voltage drop across resistor IS.

The tube I has its plate energized from a source of direct current voltage B (not shown) through a coil II". The latter coil is part of the output network which is resonated to the operating intermediate frequency. The tuning condenser for the I. F. network is designated by numeral l8, and the condenser has one side thereof connected to the plate side of coil II". The other side of the condenser is connected to ground through a path including coil l9 and condenser 29. A variable tuning condenser 2|, similar in construction to condenser 9, is connected across coil l9 and condenser 20. The coil l9 and condenser 2| comprise a circuit capable of being tuned to the same signal frequencies as the circuit 8-9 is tuned to. The coil l9 has a wave band switching device operatively associated therewith.

The tube 2 includes in its input two networks similar in construction to network |8-||" and network |92|. Thus, coil 22 is magnetically coupled to coil l1", and is connected between the signal input grid of tube 2 and ground. Condenser 23 tunes coil 22 to the operating intermediate frequency, the condenser being connected on one side thereof to the grid side of coil 22. The condenser 23 is connected to ground through coil 24 and condenser 29. The variable condenser 25, similar in construction to condensers 9 and 2|, is connected in shunt with coil 24. The network 25--24 is tunable to the same signal frequencies as network |9-2|, and coil 24 has a wave band switching device operatively associated therewith. The rotors of variable condensers 2| and 25 are arranged to be unicontrolled, as shown by the dotted lines, with those of condensers 9 and IS.

The AVC lead I is connected to the grid circuit of tube 2 through filter resistor I2 and coil 22. Of course, the adjustable taps of the wave band switching devices of the several tunable circuits may be arranged for simultaneous mechanical actuation; those skilled in the art are fully aware of .such a construction. The plate circuit of detector tube 2 includes the network 4 which is resonated to the operating intermediate frequency. By means of tuned circuit 4, coupled to circuit 4 as at M1, the I. F. energy is impressed upon the amplifier 5. The AVC lead I is connected to the signal grid circuits of amplifier 5 through filter resistor 40.

The local oscillator is connected to the first detector tube 2 by connecting the cathode side of resistor IE to the cathode of tube 2 through lead II. In operation, the tubes and 2 are used as first detectors by injecting the local oscillator voltage into their input circuits. Signal energy of a desired frequency, chosen from a selected wave band, is impressed upon detector tube However, in the output circuit of tube I there is caused to exist I. F. energy, equal to the difference of the oscillator and signal frequencies, and energy of signal frequency. In other words there is transferred from the output circuit of tube to the input electrodes of detector tube 2 energy of intermediate and signal frequency. It will now be seen that tube functions both as a. first detector and as a carrier wave amplifier. Tube 2 functions as a first detector and as an intermediate frequency amplifier.

The intermediate frequency energy output from tube 2 will depend upon both the I. F. and signal frequency input to tube 2, both as to amplitude and phase. At resonance these two inputs will be of such phase as to aid in producing the I. F. energy output in the plate circuit:

of tube 2. Off resonance, on the other hand, the phase of the signal frequency will advance, while that of the I. F. will be retarded; thus producing cancellation in the output of tube 2 when the total phase displacement reaches pi and the amplitudes are such as to give approximate equal intermediate frequency outputs from tube 2. The coils I1" and 22 should have negative mutual inductance when the signal circuits directly below them are coupled by capacity 20 as shown. If the coils l9 and 24 of the signal circuits are coupled together magnetically instead of using coupling capacity 20, then the sense of the magnetic coupling between the coils l9 and 24 should be the same as that between I. F. coils I1 and 22.

A consideration of the comparative selectivity curves shown in Fig. 2 will graphically demonstrate the utility of the present invention. The full line curve A shows the selectivity of the signal circuits from the input of tube to. the output of tube 2, when the present invention is not embodied in the receiver. However, the dotted line curve C shows the manner in which the selectivity curve of the whole system from the input of tube to the output of tube 2 is altered, when the present invention is utilized. The curve C is characterized by its steeper sides near resonance.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

l. A superheterodyne receiver comprising a pair of first detector tubes arranged in cascade, a local oscillator connected to the pair of first detector tubes to impress the locally produced oscillations thereupon, a signal input circuit connected between the input electrodes of the prior one of said first detector tubes, a network tuned to the operating intermediate frequency in the output circuit of the following of said first detector tubes, and a network coupling the output circuit of the prior detector tube to the input circuit of the following detector tube, saidlast named network comprising at least one circuit tuned to the intermediate frequency and one circuit tuned to the frequency of the signal input circuit of said prior detector tube, the constants of the circuits of said coupling network being so chosen that at resonance the signal and intermediate frequency inputs to the following one of said detector tubes are of such phase as to aid in producing the intermediate frequency energy in the output of said following detector tube.

2. A superheterodyne receiver including a first detector tube provided with a signal input circuit, an intermediate frequency output circuit and a local oscillator coupled to the first detector tube to impress iocai oscillations thereon, the receiver being characterized by the inclusion of a second tube in cascade with the first detector tube, and a coupling network between the first detector tube and the input circuit of said second tube, said coupling network including a circuit tuned to the operating intermediate frequency and a circuit tuned to the desired signal frequency, means impressing the local oscillations on the second tube, and an intermediate frequency output circuit connected to the output electrodes of the second tube. V

PAUL O. FARNHAM. 

